Firstly turn off the water supply at the tank - this will tell you if you have a leak in the supply runs. If it still leaks check around the base of the tank. Then basically it's checking every joint in the supply runs. Worst case, if you find nothing, you will have to start spending, get it out of the water and get someone with boat knowledge to look around under the waterline with you.
Also, what work has been carried out on the boat since you got it, was it lifted out for survey? (Sprung plate etc). It's a long process.
You say vast amounts - how much over night?